1. Field of the Invention
The present invention relates to a method for fabricating a capacitor. More particularly, the present invention relates to a method for fabricating a metal-insulator-metal capacitor.
2. Description of the Related Art
With great advances in technologies, semiconductor devices have found increasing number of applications. A large number of semiconductor devices each having a different function is used inside computers, communication equipment and consumer electronic products. Capacitor is one of the most basic and important semiconductor devices that has a number of functions including the de-coupling of noise and the storage of electric charges. Among the various types of capacitors, the metal-insulator-metal (MIM) capacitor plays an important role in circuits, particularly in mixed circuits of signal devices and logic devices.
The conventional method of fabricating a metal-insulator-metal capacitor includes the following steps. First, a metal layer is formed on a substrate to serve as the bottom electrode of the capacitor. Then, an oxide/nitride/oxide (ONO) composite layer is formed over the metal layer to serve as a dielectric layer for the capacitor. Thereafter, another metal layer is formed over the ONO composite layer to serve as the top electrode of the capacitor. Finally, the two metal layers and the ONO layer are defined to form the metal-insulator-metal (MIM) capacitor.
It should be noted that the permissible size of the capacitor continues to shrink with the development and increasing integration of the integrated circuits (ICs). Therefore, for an electronic product that incorporates a capacitor, the capacitor may have insufficient capacitance per unit area for proper functioning. To resolve this problem, the most common method is to reduce the thickness of the dielectric layer of the capacitor so that the capacitance per unit area of the capacitor is increased.
However, due to the unevenness of the metal layer that serves as the bottom electrode, the foregoing method of reducing the thickness of the dielectric layer above the bottom electrode may lead to lowering of the breakdown voltage of the capacitor. In some cases, even the reliability of the capacitor may be seriously compromised.